Conventionally, a gas clean-up unit including a COS converter that converts carbonyl sulfide in gas to be treated such as coal gasification gas to hydrogen sulfide, in the presence of a catalyst, has been developed (for example, see Patent Literature 1). In the COS converter of the gas clean-up unit, carbonyl sulfide contained in the coal gasification gas is hydrolyzed to hydrogen sulfide and carbon dioxide by the hydrolysis reaction represented by the following reaction formula (1).COS+H2O⇔H2S+CO2  formula(1)